Conventionally, there have been provided various position sensors each for detecting a displacement of a target object (e.g., a rotation amount, a rotation angle or a rotation position of a rotating target object) as disclosed in, e.g., Patent Document 1. A displacement sensor (position sensor) described in Patent Document 1 includes a detection coil wound around a cylindrical core formed of a non-magnetic material, and a tubular electric conductor arranged near the inside or outside of the detection coil and capable of displacing in an axial direction of the detection coil.
An oscillation circuit outputs an oscillation signal of a frequency corresponding to the inductance of the detection coil varying depending on the distance between the electric conductor and the detection coil and the capacitance of a capacitor connected in parallel with the detection coil, and the displacement of the conductor is detected based on the oscillation signal. Accordingly, the displacement of the target object can be detected by detecting the displacement of the electric conductor moving together with the target object detected based on a change in the inductance of the detection coil.
In this case, the oscillation circuit used in the position sensor as described above is required to faithfully reproduce a resonance frequency of a resonance circuit including the detection coil and the capacitor, and to be inexpensive and suitable for mass production of integrated circuits. A proximity sensor (position sensor) using such oscillation circuit is disclosed in, e.g., Patent Document 2. The proximity sensor described in Patent Document 2 will be briefly described with reference to the drawings.
The proximity sensor includes, as shown in FIG. 6, a resonance circuit 100 including a detection coil L100 and a capacitor C100, and an oscillation circuit 101 which supplies a feedback current If to the resonance circuit 100 to sustain the oscillation of the resonance circuit 100. The oscillation voltage obtained by level-shifting the amplitude of the oscillation voltage outputted from the resonance circuit 100 through an npn type transistor 102 is inputted to the oscillation circuit 101. The level-shifted oscillation voltage is also inputted to a signal processing circuit 103, and the signal processing circuit 103 switches its output depending on the magnitude of the amplitude of the inputted oscillation voltage, thereby detecting an approach of a conductor (not shown), which is a target object to be detected, to the detection coil L100.
The oscillation circuit 101 includes a current mirror circuit having two pnp type transistors 101a and 101b, and the feedback current If is positively fed back to the resonance circuit 100 by the action of the current mirror circuit. Further, the oscillation circuit 101 includes an npn type transistor 101c having a collector connected to a collector of the transistor 101b and an emitter connected to a feedback resistor Rf to configure an emitter follower. A current value of the feedback current If is controlled based on an emitter potential of the transistor 101c, i.e., a voltage applied to the feedback resistor Rf. Further, the oscillation circuit 101 is connected to an amplitude limiter circuit 104 for limiting an amplitude of an oscillation voltage level-shifted in the transistor 102 to a predetermined amplitude.
Further, a negative conductance Gosc of the oscillation circuit 101 is determined by a resistance value of the feedback resistor Rf. For example, if the resistance value of the feedback resistor Rf is R, an absolute value of the negative conductance Gosc of the oscillation circuit 101 is given by |Gosc|=1/(2R). In order to sustain the oscillation of the resonance circuit 100, it is necessary to set the negative conductance Gosc of the oscillation circuit 101 to be always equal to or greater than a conductance Gcoil of the detection coil L100.
Patent Document 1: Japanese Patent Application Publication No. 2008-292376
Patent Document 2: Japanese Patent Application Publication No. 2002-267765
In the above-mentioned conventional example, depending on the relative position between the electric conductor and the detection coil L100, the inductance of the detection coil L100 as well as the conductance Gcoil of the detection coil L100 varies. Thus, it is necessary to accommodatingly set a value of the negative conductance Gosc of the oscillation circuit 101 while taking into account the variation of the conductance Gcoil of the detection coil L100. However, it has been found from experiment that an error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 varies depending on the set value of the negative conductance Gosc of the oscillation circuit 101. This problem will be explained based on experimental results.
In this experiment, when the electric conductor was displaced in a range from 0 to 60 mm, the conductance Gcoil of the detection coil L100 was varied in a range from 200 to 900 μS. Further, the oscillation frequencies of the oscillation circuit 101 were measured when a resistance value R of the feedback resistor Rf was set to 430Ω (|Gosc|≈1.2 mS) and when it was set to 240Ω (|Gosc|≈2 mS). The results are shown in FIGS. 7A and 7B.
As shown in FIG. 7A, it has been found that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 became larger as the absolute value of the negative conductance Gosc of the oscillation circuit 101 became larger. Further, as shown in FIG. 7B, the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 has been found to become larger as the displacement of the electric conductor, i.e., the conductance Gcoil of the detection coil L100, became smaller.
From these results, it has been found that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 becomes larger as a difference between the absolute value of the negative conductance Gosc of the oscillation circuit 101 and the conductance Gcoil of the detection coil L100 is larger.
In addition, it has been found from the experiment that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 is also varied by a change in the ambient temperature of the oscillation circuit 101.
Hereinafter, this problem will be explained based on the experimental results. In this experiment, the resistance value R of the feedback resistor Rf was set to 270Ω (|Gosc|≈1.9 mS), and the oscillation frequencies of the oscillation circuit 101 were measured in cases where the ambient temperatures were 25° C. and 125° C.
As shown in FIGS. 8A and 8B, it has been found that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 became large in the case of the ambient temperature of 125° C. as compared with the case of the ambient temperature of 25° C. More specifically, in the case of the ambient temperature of 25° C., the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 has been found to have an error of 1 to 1.5%. On the other hand, in the case of the ambient temperature of 125° C., the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 has been found to have an error of 2 to 3.5%. That is, it has been found that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 becomes large as the ambient temperature increases.
As described above, in the conventional example, there has been a problem in that the error in the oscillation frequency of the oscillation circuit 101 with respect to the resonance frequency of the resonance circuit 100 is increased or varied by the setting of the negative conductance Gosc of the oscillation circuit 101 and the ambient temperature of the oscillation circuit 101.